This invention relates to shielded rooms that produce an internal environment free of external electromagnetic and magnetic fields and to the measurement of biomagnetic fields within such rooms, and, more particularly, to the construction of the rooms.
The biomagnetometer is an instrument that has been developed to measure magnetic fields produced by the body, particularly the brain. The magnetic fields produced by the body are very small and difficult to measure. Typically, the strength of the magnetic field produced by the brain is about 0.00000001 Gauss. By comparison, the strength of the earth's magnetic field is about 0.5 Gauss, or over a million times larger than the strength of the magnetic field of the brain. Most electrical equipment also produced magnetic fields, in many cases much larger than that of the earth. Electromagnetic signals travelling through the environment can also interfere with the taking of magnetic measurements. It is apparent that, unless special precautions are taken, it is difficult or impossible to make magnetic measurements of the human body because the external influences such as the earth's magnetism, nearby apparatus, and electromagnetic signals can completely mask the magnetic fields from the body.
The biomagnetometer includes a very sensitive detector of magnetic signals. The currently most widely used detector is a Superconducting QUantum Interference Device or SQUID, which is sufficiently sensitive to detect magnetic signals produced by the brain. (See, for example. U.S. Pat. Nos. 4,386,361 and 4,403,189, whose disclosures are incorporated by reference, for descriptions of two types of SQUIDs.) This detector and its associated equipment require special operating conditions such as cryogenic temperatures, and cannot be placed into the body or attached directly to the surface of the body.
The present biomagnetometer usually includes a chair or table upon which the patient is positioned, and a structure which supports the SQUID in a cryogenic environment and in proximity with the head of the patient, as about 8 inches away. Special electronics are used to filter out external effects, see for example U.S. Pat. Nos. 3,980,076 and 4,079,730, whose disclosures are herein incorporated by reference. The electronics filters out a portion of the external noise, but in some regimes is not entirely successful. The electronics is also costly and can constitute a major portion of the cost of the system.
There is another possibility for reducing the adverse effect of the external magnetic field, which can be used in place of, or in addition to, the electronic signal processing. In this approach, the patient and detector are placed into a magnetically quiet enclosure that shields the patient and the detector from the external electromagnetic and magnetic fields. The magnitude of the Earth's static magnetic field within the enclosure is reduced from about 0.5 Gauss or more, to less than about 0.001 Gauss. With this reduction in the ambient magnetic field, the biomagnetic events of interest can be measured more readily, and the signal processing required to achieve usable information is greatly reduced.
Magnetically shielded enclosures have been known, as for example the design described in U.S. Pat. No. 3,557,777, whose disclosure is herein incorporated by reference. In this approach, concentric layers of a high permeability metal and a metallic conductor are supported on a frame, thereby forming the walls of the shielded room. To permit construction of the room at remote sites, it is conventional practice to provide the high permeability material and the metallic conductor material as sheets that are assembled to the frame. The '777 patent indicates that the layers of shielding sheets are simply fastened to a wooden frame with screws. While this practice may have been sufficient with the biomagnetic measurement technologies available in the 1960's, current practice with better biomegnetic measurement equipment requires that the interfaces between the sheets must be sealed more positively to prevent field leakage to the interior of the shielded room.
In current construction practice, the edges of the sheets of metallic conductor material are welded to each other to form a continuous shielding surface after assembly to the frame. The welded construction avoids the possibility of leakage of electromagnetic energy through gaps between the sheets, as even a slight leakage can significantly interfere with the current biomagnetic measurements. The sheets of high permeability metal are assembled with large overlaps between the sheets and mechanically fastened, or assembled with small overlaps between the sheets and mechanically clamped. This construction is intended to prevent the leakage of the external magnetic field around the edges of the sheets.
While operable, enclosures having such a partially welded construction cannot be readily disassembled for movement at a later time, as to a new facility. The welding operation must be carefully performed and checked, so that the preparation of each such enclosure is essentially a custom operation, requiring long lead times.
Accordingly, there exists a need for an improved magnetically shielded enclosure which has a low level of electromagnetic and magnetic noise in its interior, the low levels being retained over extended periods of time. Such an enclosure should be capable of being disassembled if necessary, and should be less expensive to construct than existing enclosures. The present invention fulfills this need, and further provides related advantages.